With the rapid growth of emerging bandwidth-demanding network services, next-generation dense wavelength division multiplexed (DWDM) optical transport technologies employing multilevel modulation formats are highly desirable to deliver information bits as many as possible over existing band-limited ITU-T channels. Polarization multiplexing return-to-zero quadrature phase shift keying (PolMux-RZ-QPSK) with digital coherent detection has been recognized as the next optical transport network standard that mitigates optical link impairments by multiplexing data tributaries at a lower bit-rate so as to be handled easily by DSP-orientated coherent receivers.
Delivering DWDM signals with more modulation levels through legacy dispersion-managed networks will suffer from the significant nonlinear penalty. One approach to reduce the nonlinearity impairment is to offset the x-polarized and y-polarized time pulses by half of the symbol rate, which is also known as “temporal polarization interleaving” (TPI) that gives a lower peak transmitting power over the entire time space. Although it is possible to include the function of TPI into a PolMUX-RZ-QPSK optical transmitter by adding an additional optical time delay along either the x-polarized or the y-polarized polarization maintaining (PM) waveguide/fiber during the fabrication, many established and upcoming PolMUX-RZ-QPSK products from different vendors for 100 Gbs Ethernet applications do not have such non-standardized functionality.